This invention relates to an ink jet printer for forming an image by selectively discharging ink by electrostatic means.
In a prior art ink jet printer, ink drops are selectively discharged by electrostatic force and attached to a recording medium to form dots, the image being formed as a selective combination of these dots. In such an ink jet printer, the dot diameter is fixed, so that it is impossible to provide a tone gradation to the image with an ordinary method. A dither method is employed as means for providing tone gradation to the image. In this method, one picture element is divided into a plurality of matrices, and dots are selectively formed in the matrices. In this case, the shade of the image is varied by varying the dot density in each picture element. When it is desired to obtain a high dot density image the number of dots formed in the picture element is increased. On the other hand, to obtain a low dot density image, the number of dots in the picture element is reduced. In this way, a tone gradation of the image is provided.
To obtain a natural or high quality image, it is necessary to increase the range of tone gradation. However, since the dot diameter is fixed, the number of matrices allotted to each picture element has to be increased to increase the range of tone gradation. This increases the size of the picture element and reduces the resolution of the image. Therefore, it has been impossible to obtain natural or high quality images by increasing the number of matrices in a picture element.